Full-width Post template problem (modified Twenty Twelve)

Hi,
I’ve been working on a website (https://www.craigsharris.com), modifying the Twenty-Twelve theme (yes, using a child theme!). I followed a nice tutorial to make the main page of the site three columns: https://zeaks.org/2012/twenty-twelve-theme-three-column-layout. The problem is – it seems to have applied it to ALL of the pages and Posts on the site. This isn’t a huge issue for the pages, as I was able to select the default Twenty-Twelve Full-width no-sidebar template for the pages, but on the Posts, when I try to apply the Full-width template (after following tutorials on the forums for creating a custom Post template), it removes the sidebars, however the Post content is all squished in the center column (even though the right and left sidebars are gone). So it looks very odd! I also tried just remove <?php get_sidebar(); ?>, as suggested on this post, but it had the same result. (Although I was unable to locate the #content in the CSS as mentioned in that Post). I’ve spent about 7 hours now searching the forums and the web trying to figure this out, so if anyone can help, I’d be very very grateful!